The invention relates to silicone-free release films having low force release characteristics which are made of thermoplastic materials, especially substantially linear ethylenic polymers and copolymers. The invention includes pressure-sensitive adhesives in the form of solid and foamed films, sheets, and tapes produced using the release films as release liners. The invention is particularly useful in the production of release films, release liners, non-stick carrier webs utilized as the casting surface for adhesives and foams, and coatings for paper and polymer liners for adhesives.
A release film or release liner is a component of a laminated film construction, e.g., pressure-sensitive adhesive foam film, which functions as a protective carrier for the film during handling and storage of the film prior to use. It is separated from an adhesive film and/or face stock of a laminated film construction prior to the application of the film to a substrate. The many types of release liners and films produced in industry may be categorized according to the peel force required to separate the liner from a pressure-sensitive adhesive coated tape under specified test conditions. The most desirable release films for pressure-sensitive adhesives (e.g., adhesives which are tacky upon mechanical contact, without thermal or chemical activation) have a peel test release force value of less than 39 g/cm (0.22 lbs/inch) in an adhesive industry standard peel test. Release liners and films for less demanding applications, such as thermally or chemically activated adhesives, or facestock for labels or printed materials or surfaces requiring protection during shipping and handling, may have release force values much higher, e.g., 55 g/cm (0.308 lbs/inch) and up to peel values typical for low strength adhesive materials.
Conventional thermoplastic materials typically exhibit higher adhesive release force values than are acceptable for pressure sensitive adhesive release films. For example, medium density polyethylene (MDPE) film known as MRLX from Atlantis Plastics has an average force-to-release in the 71-89 g/cm (0.4-0.5 lb/inch) range. Similar, undesirable, high release force values are disclosed in U.S. Pat. Nos. 4,339,485 and 4,425,176 for low density ethylene/alpha-olefin copolymers and blends of these copolymers with medium density polyethylene used as release liners for pressure sensitive acrylic adhesive articles. High pressure sensitive adhesive release force values are disclosed for a broad range of low density polyethylene and alpha-olefin copolymers in JP-A-94-99551, JP-A-93-329985 and JP-A-93-286096.
A new class of low density polyolefins manufactured utilizing metallocene catalysts ("metallocene polymers") are inherently tacky, blocky and self-adherent materials and would be expected to yield even higher release force values from pressure sensitive adhesives than the conventional polyolefins described above.
Commercially used premium, low force release films for pressure sensitive adhesives presently are fabricated from fluoropolymers or silicone polymers, or by using silicone coatings on support materials such as polyester or paper. See, for example, U.S. Pat. No. 5,380,572, and U.S. Pat. No. 5,451,440. The fluoropolymers, fluoroethylene propylene copolymer (FEP) and polytetrafluoroethylene (PTFE), typically exhibit force-to-release values from pressure-sensitive adhesives in the 14-27 g/cm(0.08-0.15 lb/inch) range, while silicone coated polyester (Mylar) has an average force-to-release value in the 9-18 g/cm (0.05-0.10 lb/inch) range.
These release film materials are effective, but have certain deficiencies. Silicone materials migrate into the adhesive and are carried over into the finished product assembly containing the adhesive where the silicone may reduce the effectiveness of the adhesive, interfere with the application of paint or other coatings and components, or present defects upon storage or use of the final product assembly. The cost of FEP and PTFE is a deterrent to their use as disposable liners in adhesive film production. These fluoropolymers are typically used as release films only in expensive specialty applications requiring very high temperature tolerances.
Other materials suggested for release films are primarily intended for release from non-adhesive surfaces and include, for example, the polymethylpentene materials described in U.S. Pat. No. 5,080,979 for use as a coverlay in the manufacture of printed circuit boards for electronic components. Another example of a non-adhesive surface release material, a carboxylic anhydride cross-linked ethylenic polymer which leaves a clean surface after peeling, is disclosed in JP-A-85-245549.
Copolymers of acrylate esters are suggested in U.S. Pat. No. 5,393,608 for use with pressure sensitive adhesive tape, but no release force values are provided.
It has been discovered that a selected class of linear, low density ethylenic polymers and copolymers characterized by a uniform molecular distribution have excellent low force release film properties and may be used as economical, disposable release liners in adhesive film production without a silicone coating. This class includes certain of the new metallocene polymers known to be tacky, blocky and self-adherent. Thus, the low release force values from pressure sensitive adhesives described herein were entirely unexpected for this class of polymeric materials.